Silicon concentration and pH controls over competitive or simultaneous incorporation of iodate and chromate into calcium carbonate phases

Highlights: • The presence of CrO₄²⁻ has little effect on IO₃⁻ removal. • Removal of IO₃⁻ is pH and Si concentration dependent. • Si controls or slows down CaCO₃ phase transformation. • More CrO₄²⁻ than IO₃⁻is released during CaCO₃ dissolution. Co-precipitation of iodate and chromate with calcium carbonate (CaCO₃) minerals is a potential approach that can significantly decrease their mobility in the subsurface. There is a need to identify the impact of pH and elements of natural abundance such as Si on co-precipitation process to understand potential for natural attenuation or targeted removal via injection of CaCO₃ forming solutions in the subsurface. Batch experiments were combined with solid phase characterization studies to examine the potential for co-precipitation/incorporation of iodate and chromate into CaCO₃ minerals. These experiments considered uptake during and after precipitation of CaCO₃ phases and measured the release of contaminants during dissolution. Experiments conducted at variable pH (6.5, 8.0 and 9.0) and Si concentrations (0 mM, 0.5 mM and 20 mM) showed that the incorporation of iodate was pH dependent and was favored at lower pH and higher Si concentration within 7 days. The presence of chromate had little effect on the removal of iodate except at pH 6.5 where iodate uptake increased 6–10% due to the formation of dietzeite, Ca₂H₂O(IO₃)₂(CrO₄). More aqueous iodate was removed in all experiments compared to chromate except some of the investigated conditions (Si 20 mM, pH 9) where the removal of CrO₄²⁻ was found similar to IO₃⁻. The release of iodate and chromate from dissolving CaCO₃ was also pH and Si concentration dependent. The I fraction released from CaCO₃ doped with IO₃⁻ was 1.5–2.3%, which reached a steady state after 24h; however, 2.6–4.6% of I was released from CaCO₃ doped with IO₃⁻ and CrO₄²⁻ for all tested conditions. More chromate (up to 36.5 ± 4.6%) than iodate (3.2 ± 0.07%) was released during dissolution at pH 6.5 and 20 mM Si. Although less uptake of Cr occurred, it may have led to increased release of I when both were co-precipitated simultaneously.